Through the looking glass: The collective Internet has come up with its own theories as to why Apple might have wanted a retractable bumper but none of the leading arguments are very convincing. Given Project Titan's current status, however, we may never know what Apple intended.
Apple in late 2015 filed a patent for extendable automotive bumpers, presumably as part of its Project Titan self-driving project. The patent was published Tuesday and describes a unique take on the traditional vehicle bumper although the goal of the patent isn't entirely clear.
The patent describes a retractable bumper that can expand and contract via an inflatable structure. It sounds interesting enough and although Apple probably had some solid reasoning behind why it should exist, we aren't privy to that information.
Some argue that a retractable bumper could be used to shorten a vehicle's overall length, saving a few inches in a parking lot or perhaps even impacting its turning radius. Neither of these are really all that important in the grand scheme of things and almost certainly aren't worth the trade-off with regard to what it would cost to implement the feature.
If Apple were truly concerned about these things, wouldn't they just make the vehicle shorter to begin with?
One could also argue that an extendable bumper may help improve safety during low-impact collisions, perhaps for pedestrians. Google filed a patent for an external air bag of sorts in 2013 for its self-driving vehicles to reduce injury to a person struck by one of its vehicles.
These are only a couple of the possibilities Apple may have been thinking of but given Project Titan's reportedly narrowed scope, it may not matter all that much today. After all, most patents never make it into products that see the light of day.